Need some advice about siblings

My step daughter has been having issues with her sister's at her mom house. They are fairly young still oldest is 7 going on 8 this year(she is my step daughter's step sister) my step daughter is 6 and her younger sister is 4. She shares a room with the 4 year old and daughter is having issues sleeping at night because her younger sister keeps her up at night causing my step daughter to be very cranky and not wanting to wake up for school when she is supposed to get up. She is also saying that she wants to stay with us because her sisters are mean to her. The youngest hurts her (hitting biting taking things from my step daughter) and the oldest is always making everything a competition saying she is better because she is older and she is smarter. This has led my step daughter to wanting to stay with us more instead of going back with her mom. We are perfectly ok with her wanting to stay and have more time with us. But we have tried telling her that she cannot avoid her sisters and she needs to be with her mom and sisters as well. I have tried telling her that when there is a problem she needs to talk to her mom or step dad or grandma(whoever is there watching them) so the problem can get solved and she says she does but the other girls still dont listen and she is still getting hurt physically and her feelings and still doesnt want to go to her moms when she is with us. We have told her mom what the issues are so she can take care of it which we think she is doing. But all we hear from my step daughter when she is with us is that her younger sister is mean and she wants to stay with us. Her younger sister is pretty tough and hard to handle where as my step daughter is more gentle girly girl quite and shy. How would you handle explaining to a child that they might not always get along with their siblings but they still need to spend time with them and be with their family and she cant use us as a way to avoid her family? My SO and i are ok with keeping her on our time but my step daughter wants us to keep her on her moms time as well and we have told her that she has days with us and days with her mom and that her mom will miss her if she doesnt go see her and spend time with her. It just doesnt seem to be working. All my step daughter says is she wants to sleep at our home so she doesnt have to deal with her sister and her sister can not hurt her when she is with us.

Also to note that my SO has mediation the end of this month for 50/50 custody legal and physical. His ex was not willing to give him that but since these issues have been happening for the past 2 months now and she is having difficulties with my step daughter she is actually considering giving him what he is asking because my step daughter is wanting it and she doesnt want to go to court.